Multistrike ribbons are used in connection with dot matrix, daisy wheel, and other impact printers, and are usually packaged in cartridges that are characterized by the number of impacts that can be obtained. Such number depends on various factors including the volume of ink on the ribbon in the cartridge, and the transfer characteristics of the ink/ribbon base combination. Knowing the cost of a cartridge and the number of characters that can be printed, one can compute the cost per impact which is a factor of considerable importance to a user in comparing cartridges.
When a print pin of a printer impacts a ribbon, the change in momentum of the ink in the ribbon due to the impulse of the print pin induces a flow of ink from the ribbon to the paper. If the ink is Newtonian (i.e., its viscosity is independent of the rate of shear), the viscosity of the ink, or its resistance to flow, is independent of the impulse applied to the ribbon by the print pin. Conventionally, however, inks used in multistrike ribbons are thixotropic (i.e., their viscosity changes inversely with rate of shear as shown in FIG. 1). At rest, prior to being struck by a print pin, the ink in the ribbon is viscous and does not ooze from the ribbon. 0n being struck by a print pin, the ink is subject to shear becomes less viscous and flows easily from the ribbon to the paper.
Multistrike ribbons are conventionally made using a carrier that includes either a plastic film or woven fabric. If the carrier were to include only a plastic film coated with thixotropic ink, all or nearly all of the ink on the film, within the projected area of a print pin, would be transferred from the film to the paper when the ribbon is struck by the print pin. The result would be a ribbon that lacks multistrike capability. Conventionally multistrike ribbons in which the carrier includes a plastic film are therefore provided with a porous layer on the side of the film facing the paper. Such layer has a large number of interstices that are filled with ink, only a measured amount of which is transferred to a piece of paper when the ribbon is struck. The structure of the layer serves to meter the flow of ink allowing multiple strikes on the same place on the ribbon to produce ink deposits on the paper.
Typically, the film is polyester and is coated with an open "sponge" of polymeric material filled with ink. In general, the polyester body is 10-30 microns thick and the "sponge" is from 8-50 microns thick with openings about 10 microns in diameter. These openings act to modify the flow of ink during impact thus metering the amount of ink transferred in response to striking the ribbon, enabling the ribbon to continue to transfer ink when struck a number of times in the same spot.
The deficiencies of this type of ribbon include reduced density uniformity for letters printed on previously struck areas of the ribbon and rapid degradation of the density and sharpness of the printed characters, the added cost of the sponge layer, and the large volume of cartridge taken up by the structure of the sponge. Other conventional multistrike ribbons use special fabrics that are woven to provide interstices for holding ink. Crimped nylon and silk are the materials presently favored for this purpose. The deficiency with this arrangement is the cost of the fabrics, the volume taken up by the ribbon base for a ribbon of this type, and the low multi-strike print quality.
It is therefore an object of the present invention to provide a new and improved ink for a multistrike ribbon which increases the number of usable characters printed by the ribbon and reduces the cost per impact.